<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<h1>0</h1>
<script>
  let count = 0;
  let timer1 = setInterval(f,100);
  let h = document.querySelector("h1");
  function f() {
    count++;
    h.innerText = count;
    if(count==100){
      clearInterval(timer1);
    }
  }
  //匿名方法开启定时器
  let timer = setInterval(function () {
    console.log(count);
    if(count==5){
      clearInterval(timer);
    }
  },1000)

  //开启只执行一次的定时器
  setTimeout(function () {
    alert("时间到!")
  },3000);





</script>
</body>
</html>